Imaging an object with a coherent beam of electromagnetic radiation is often plagued with the phenomenon of speckle, i.e., the randomly bright and dark grainy appearance of an image resulting from alternately constructive and destructive interferences over the aperture and the field of view. If the electromagnetic radiation source is perfectly coherent, the severity of the speckle will depend on three dimensionless parameters: the root-mean-square object roughness in terms of wavelengths, the average lateral spacing between surface features in wavelengths, and the number of features in the illumination area. Many kinds of laser illumination, particularly holography, invite the speckle problem because the number of features within the illumination area whose height deviates from the average height by approximately half an optical wavelength or more is enormous. In contrast to coherent imaging, incoherent imaging is immune to speckle since any set of optical paths from the source to the object to the detector (e.g., a retina, film, a CCD array) is only destructive for a fraction of the wavelengths involved.
Some techniques for dealing with speckle in coherent imaging systems utilize frequency diversity or angle diversity to reduce the occurrence of alternating constructive and destructive interference. Although these techniques can reduce speckle, the usefulness of frequency diversity is limited by a scarcity of spectrum in certain microwave regions of interest and the usefulness of angle diversity is limited by mechanical and cost considerations.
In view of this, what is needed is a coherent imaging technique that reduces speckle and that is efficient to implement.